Constant volume pressure pick-up



July 29, 1958 J. A. ROMAN 2,845,960

CONSTANT VOLUME PRESSURE PICK-UP Filed April '1, 1954 FIG. 2

:i D vi a? i" v INVENTOR. JAMES A. ROMAN BY ATTORNEYS,

pas

CONSTANT VOLUME PRESSURE PICK-UP James A. Roman, Cleveland Heights, Ohio Application April 1, 1954, Serial No. 420,296

1 Claim. (Cl. 1282.05)

This invention, relating as indicated to a constant volume pressure pick-up, is particularly directed to a blood pressure recording device.

The measurement of human arterial blood pressure is at present mainly of two types. One type involves surgical access to a blood vessel. The advantage of critical accuracy attainable by this method is offset by the risk inherent in the surgical procedure and its inadaptability to routine measurements. The other type is more practical in that it takes up little time and does not require surgical access to a blood vessel. It does, however, suffer fro-m two serious limitations:

(1) It indicates only systolic and diastolic pressures, providing no information concerning pressure-time relationships at any other pressures.

(2) Rather than providing a record of pressures for every heart beat, this method gives what amounts to average systolic or diastolic pressures over a range of several beats. Such information is meaningful only as far as it can be related to heart action which is regular both in rhythm and in stroke volume.

The object of this invention is to describe a new general method by which the measurement of blood pressures may be significantly improved both with regards to the attainment of more complete information covering each heart beat and with regards to eliminating the necessity for surgical access to a vessel to derive such information.

To the accomplishment of the foregoing and related ends said invention, then, consists of the means hereinafter fully described and particularly pointed out in the claim, the following description setting forth in detail one approved means of carrying out the invention, such disclosed means, however, constituting but one of the various ways in which the principles of the invention may be used.

In the drawings:

Fig. 1 is a cross-sectional view of this improved constant volume pressure pick-up,

Fig. 2 is a view along the lines 2--2 of Fig. 1 of said apparatus, and

Fig. 3 is a schematic view of the apparatus.

In the following it will be shown that if any vascularized part of the body is surrounded by a relatively incompressible fluid and enclosed in a rigid container, then the pressure of the fluid, as measured by a suitable instrument, will be closely equal to the arterial'blood pressure in the enclosed part. It will also be shown that the use of an incompressible fluid is a condition sine qua non for blood pressure measurement by this method, for only by use of such a fluid may the enclosed part of the body be prevented from undergoing changes in volume during the cardiac cycle.

By preventing these volume changes, the blood vessels surrounding tissues and skin are prevented from being expanded by forces due to blood pressure. Thus the vessels, surrounding tissues and skin cannot develop tensile stresses as a reaction to the pressure of the blood States Patent contained within them. As a result of this the pressure of the blood, instead of being balanced partly by tensile stresses in the vessels, surrounding tissues and skin and partly by the pressure outside the part is now balanced by the pressure outside thepart only. In a constant volume system, therefore, the pressure of the fluid surrounding the part is the pressure of the blood in that part.

In the drawings, Fig. 1 shows the container at 10 and 11 is the upper extremity of an arm having a forearm 12 extending within the container. In the container Wall 13 is an opening 14 having a flexible membrane 15 sealed to the container wall and suitably shaped for admission of the arm. In addition there are inlet means for fluid at 16 and outlet means at 17. The container itself is rigid and has support means for a portion of the body shown at 18, and an accurate pressure instrument shown at 19.

At the opening to the container 14 there will be support means 20 having a socket arrangement for the elbow to hold the forearm in fixed position with respect to the container. Within this membrane 12 a portion of the body, such as the arm, is inserted, and the arm is hermetically enclosed within the rigid container 10 except for its connection to the body. The incompressible fluid within the container 10 would be water, or similar fluid. No air should be trapped between the arm and the membrane.

An analysis of a typical constant volume system follows. As is customary in a limited analysis of this type, certain simplifying assumptions are made for the sake of expediency. A constant volume system will be compared to a non-rigid system in order that essential points about the former may be made clear.

Let us assume a system (Fig. l) in which an arm C is hermetically enclosed in a rigid container D except for its connection to the body. The medium M between the arm and the container is water or a similar fluid in the case of the constant volume system and air or a similar fluid in the case of a non-rigid system. For purposes of analysis, the system made up of the arm, medium and container may be reduced to a simpler equivalent system. The tissues of the arm, including the vessel walls, possess elastic characteristics, i.' e. they will tend to resume their original shape after a deforming load has been removed. For purposes of this analysis, therefore, the blood vessel Walls and surrounding tissues will be regarded as one single flexible elastic membrane of modulus of elasticity in tension E. The resultant system is represented in Fig. 3. The flexible membrane has been given spherical shell shape in order to permit analysis on a mathematically simple plane. The pressure in Brepresents the absolute pressure of the blood Within the vasculature of the enclosed part of the body. The pressure P represents the pressure of the measuring fluid (medium) outside the arm and is read on a suitable instrument I, here represented as a gauge for purposes of simplicity.

, The foregoingsimplifications are for the sake of mathematicalexpediency. It can be shown that the general principles derived from an analysis of the reduced system also hold for the more complicated systems of body parts.

Let:

P P =the pressure difference between G and M,

force/unit area.

D=sphere mean diameter, units of length.

e=strain of membrane, units of length/unit length.

c=circumference of sphere, units of length.

c =original circumference of sphere, i. e. when P P equals 0, units of length.

v=volume of sphere, units of length cubed.

V =volume of sphere when P -P equals 0, units of length cubed. =tensile stresses in one direction in membrane, force per unit area.

n==-Poissons ratio for the membrane material, pure number.

t=thickness of membrane, units of length.

E=modulus of the membrane in tension, force/unit area.

The tensile stress in the sphere is:

The circumference of the sphere is:

The strain, taking into account Poisson's ratio for the membrane material, is:

Let us assume that in the constant volume system the volume of M has been adjusted so that v: V at diastolic pressure or below. Volume M, by virtue of the construction of the apparatus, cannot change. Therefore, v always equals V and the ratio v/V is always equal to 1. According to Equation 8 therefore, P P,,,=0 01 P =P This means that the pressure of the measuring fluid will always equal blood pressure,

Regardless of E, the elasticity of arteries, surrounding tissues and skin,

Regardless of D, the diameter of the part being measured,

and

Regardless of t the thickness of the vessels Walls,

surrounding tissue and skin (within limits).

Throughout the discussion simultaneous use of a suitable dynamic or maximum-minimum pressure registering device is implied, by means of which P may be read or recorded, depending upon the type of information required.

In the non-rigid system, where air is the medium, v is not equal to V except, at most, at one single instant in 6 the heart cycle regardless of the manner in which volume 4 M was originally adjusted. Therefore, the pressure I is dependent not solely upon P as in the constant volume system but also upon E, upon t, upon V and upon D. In the case of sympathetic or parasympathetic stimulation, when the musculature of the vessels of the arm either relaxes or contracts, E is correspondingly changed and P shows a change which bears no relation to changes in blood pressure. Vascular changes due to arteriosclerosis of any type will also affect E. It is seen that the difference in pressure between blood and measuring fluid is affected by the volume and thickness of the body part enclosed.

Let us suppose that in the subject being tested, parasympathetic stimulation causes widespread relaxation of the arterial musculature. Blood pressure will probably drop, at least temporarily. But since E decreases because of the relaxed musculature, P according to Equation 8, might actually increase while the pressure of the blood is decreasing. The inverse can be shown to be possible with sympathetic stimulation. Thus, even when calibrated for the particular v, D, and t of a certain patient the nonrigid system can be depended upon to be neither quantitatively nor qualitatively reliable.

By Way of contrast, a constant volume system was shown to be insensitive to changes in either E, v, D, or I.

These conclusions were arrived at by assuming that the tissues in the arm possess a modulus of elasticity E which need not be constant and by assuming further that the blood supply of the arm acts for purposes of pressure measurement as if it were concentrated in one single large channel in the center of the arm. Limited experimental Work done up to this writing indicates that these are workable assumptions. The venous return of the part of the body being used to measure blood pressure being occluded by the pressure of whatever seal is used to retain the measuring fluid, no pressure drop occurs between arterial and venous sides and the whole vasculature of the part being tested is at arterial pressures. In the case of non-rigid systems this could lead to petechiae or larger hemorrhages. In the case of a rigid system there is no danger of either, since both the measuring fluid pressure and the tissue pressure are equal to arterial pressures.

I wish it to be understood that I do not confine myself to the precise details herein set forth in the preferred manner of carrying out my invention, as it is apparent that many changes and variations may be made by those skilled in the art without departing from the spirit of the invention or exceeding the scope of the appended claim.

I claim:

A blood pressure recording apparatus comprising a rigid container, an opening in said container through which a portion of the body may be admitted, incompressible fluid in said container, a flexible diaphragm connected to the opening in said container, shaped in the form of the portion of the body over which it is adapted to be placed, and pressure responsive means attached to the wall of the container for recording variations in pressure of the incompressible fluid means, means on said container for retaining a portion of the body in constant volume relationship with the container.

References Cited in the file of this patent UNITED STATES PATENTS 

